The term "microwave assisted chemistry" refers to the use of electromagnetic radiation within the microwave frequencies to provide the energy required to initiate, drive, or accelerate certain chemical reactions. As chemists have long been aware, the application of heat energy is one of the most significant factors in increasing the rate of a wide variety of chemical reactions. Thus, generally familiar devices such as the Bunsen burner, other types of gas burners, hot plates, and other similar devices have historically been used to initiate or accelerate various chemical reactions.
As a relatively crude comparison, microwave assisted chemistry techniques are used to heat chemical reagents in the same way that a consumer microwave oven cooks food. There are significant differences, however, between the ordinary consumer use of microwave energy with food and its laboratory use with chemical reagents. Thus, the devices and techniques required for microwave assisted chemistry are generally much more sophisticated than are the consumer-oriented devices and techniques.
In one comparison, however, a laboratory microwave device and a consumer microwave offer the same advantage: in many circumstances they both greatly increase the rate at which materials can be heated as compared to the rates that they could be heated by ordinary conduction or convection heating. Thus, microwave assisted chemistry has been particularly valuable in driving or accelerating reactions that tend to be time-consuming under more conventional heating techniques. Particular examples include moisture analysis, in which samples must effectively be heated to dryness; digestion, a process in which a chemical composition is broken down into its elements for further analysis, with the breakdown generally being accomplished by heating the composition in one or more mineral acids; and the Kjeldahl techniques for nitrogen determination. Using conventional heating techniques, moisture analysis, Kjeldahl, or digestion reactions can be very lengthy, extending for hours in some cases. When the reactions are microwave assisted, however, they can be completed in a much shorter period of time. It will be understood that this time savings has a particularly significant advantage in any situation in which large number of samples must be tested on an almost continuous basis. Thus, although microwave assisted chemistry is relatively new compared to some other techniques, it has become well established and accepted in a number of analytical applications.
As well understood by those familiar with the electromagnetic spectrum, the term "microwave" is often used generically to refer to radiation with wavelengths of between about 1000 and 500,000 microns (.mu.), and corresponding frequencies of between about 1.times.10.sup.9 and 5.times.10.sup.11 Hertz (Hz). These are arbitrary boundaries, however, and other sources refer to microwaves as having frequencies of between about 10.sup.8 Hz and 10.sup.12 Hz and wavelengths of between about 300 centimeters (cm) and 0.3 millimeters (mm). For commercial and consumer purposes in the United States, the available microwave frequencies are regulated by the Federal Communications Commission and are generally limited to selected frequencies. Because of the relatively long wavelength of microwave radiation, microwave assisted chemistry techniques are often carried out in closed vessels which are in turn placed inside a device that bears a superficial relation to a consumer microwave oven, but that is much more sophisticated in its source, waveguide, cavity, and control elements.
Although the simple application of microwave energy to devices in sealed vessels has some advantages, the technique becomes particularly useful when the reactions inside the vessels can be monitored while microwaves are being applied. Thus, in a typical microwave assisted chemistry system, a plurality of similar reactions are carried out at the same time in separate closed vessels that are placed together in a single cavity and then concurrently exposed to microwaves from a single source. Typically, one of the vessels carries temperature and pressure measuring devices. This "sensor vessel" is monitored and the conditions therein are assumed to be representative of the conditions in the remainder of the vessels to which microwaves are being applied.
Stated differently, in certain microwave assisted systems, a group of reaction vessels (typically six or eight) is placed into the microwave device at the same time, and often on a turntable that rotates as the microwaves are being applied. As noted above the wavelength of microwaves is typically larger than the items being heated, so that stationary items are not always evenly exposed to the microwaves. Accordingly, smaller items such as reaction vessels and relatively small amounts of chemical reagents are best moved on a periodic basis while being exposed to the microwaves. For similar reasons, consumer kitchen microwave ovens typically include fan-like stirrers to more evenly reflect microwaves within a cavity, or turntables for rotating food as it cooks. Alternatively, microwave cooking instructions typically tell the consumer to turn, stir, or otherwise move the food during the cooking process.
By monitoring the temperature and pressure of the sensor vessel, the application of microwave power, either in terms of energy level or time, can be adjusted on a regular basis based on the feedback from the sensor vessel. Because in typical applications a plurality of vessels are rotating on a turntable, however, attempting to monitor every single reaction vessel becomes at least unwieldy, or at worst impossible. Thus, if the reaction inside one of the unmonitored vessels differs significantly from the reaction inside the monitored vessel, the reaction in one or more of the unmonitored vessels can proceed in quite undesirable fashion. As a result, the reactions in the unmonitored vessels can proceed either too far or not far enough to accomplish their intended purpose. Such results can significantly reduce the advantages of the microwave assisted technique.
An ideal method of measuring the temperature of ongoing chemical reactions in a plurality of separate vessels is to attempt to place a high-quality temperature sensor such as a thermocouple in each vessel and connect all of the thermocouples to appropriate electronic circuitry and memory that can convert the thermocouple signal into a temperature reading.
To date, however, individual temperature measurement from inside the vessels has remained somewhat unwieldy for several reasons including the movement of the vessels in the cavity, for example on a turntable as noted above. This movement greatly increases the complexity of physical connections between the individual moving vessels and the remaining stationary portions of the device.
Furthermore, in microwave assisted techniques, external temperature measurement also presents certain problems. In particular, the reaction vessels used in microwave assisted chemistry, particularly closed reaction vessels, must be formed of materials that are transparent to microwave radiation, resistant to chemical attack, and strong enough to withstand high internal gas pressures. Typically, the vessels are formed from certain types of engineering polymers that offer these properties. Although the microwave transparency of such materials is a great advantage, such polymers tend to have relatively poor heat conductivity and instead act as insulators. Thus, as opposed to vessels intended for external temperature measurement (such as the stainless steel "bomb" of a calorimeter), the external temperature of vessels for microwave assisted chemistry tend to differ widely from the actual internal temperature. Although some of these polymer materials will eventually reach an equilibrium temperature with their contents, the time required to reach such equilibrium temperature ("lag time") can be longer than the progress of many chemical reactions taking place under microwave assistance. Thus, the temperature feedback from the exterior of a microwave assisted chemistry vessel is often unsatisfactory for analytical or control purposes.
Additionally, external measurements carried out by radiometric techniques can be affected by the changes in thermal emission properties of the vessel material that follow a change in temperature. Stated differently, for some materials the change in the black body radiation they emit does not linearly follow the change in temperature. These nonlinear relationships can add to the complexity of external temperature measurement.